Bimetallic catalyst for nox  reduction in engine exhaust

ABSTRACT

A catalyst made by the introduction of copper ions and cobalt ions into a Y-type zeolite to produce a CuCoY zeolite improves the reduction of NOx in exhaust from a lean burn engine, such as a diesel engine. A dual bed selective reduction reactor in the exhaust stream comprising a first bed of BaY or of Ag/alumina and a second bed of CuCoY is particularly effective in reduction of NOx. In one embodiment, the exhaust stream is suitably treated with an oxidant, such as ozone, and a reductant, such as a diesel hydrocarbon or oxygenated diesel hydrocarbon, before the exhaust passes in contact with the dual bed catalyst.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 11/853,262filed Sep. 11, 2007.

TECHNICAL FIELD

This disclosure pertains to catalysts for use in NOx reduction in leanburn engine exhaust using hydrocarbon reductants. More specifically,this disclosure pertains to the use of new bimetallic catalysts in suchan application.

BACKGROUND OF THE INVENTION

Diesel engines and other lean-burn engines or power plants are operatedat higher than stoichiometric air-to-fuel mass ratios for improved fueleconomy. Such lean-burning engines produce a hot exhaust with arelatively high content of oxygen and nitrogen oxides (NO_(x)). Thetemperature of the exhaust from a warmed-up diesel engine is typicallyin the range of 200° C. to 400° C. and has a representative composition,by volume, of about 10-17% oxygen, 3% carbon dioxide, 0.1% carbonmonoxide, 180 ppm hydrocarbons, 235 ppm NO_(x) and the balance nitrogenand water. These NO_(x) gases, typically comprising nitric oxide (NO)and nitrogen dioxide (NO₂), are difficult to reduce to nitrogen (N₂)because of the high oxygen (O₂) content in the hot exhaust stream.

U.S. Pat. Nos. 6,957,528 and 7,093,429, and U.S. Patent ApplicationPublication 2006/0283175, each assigned to the same assignee as thisinvention, disclose methods of adding ozone and nonthermalplasma-reformed diesel fuel constituents to the exhaust stream flowingfrom a lean burn engine or power plant preparatory to selectivecatalytic reduction (SCR) of NOx. Ozone is added to the exhaust streamfor oxidation of NO to NO₂. And air/ozone plasma-generated, lowmolecular weight, oxygenated hydrocarbons and hydrocarbons from afractionated portion of the diesel fuel hydrocarbon mixture are added tothe exhaust as reductants for conversion of NO₂ to N₂ over a selectivereduction catalyst. This process may be called diesel fuel/SCR. Cu/Yzeolite and Ag/alumina catalysts have been considered for use in SCR.However, there remains a need for improvement in the conversion of NOxin diesel exhaust and in the exhaust of other lean burn engines.

SUMMARY OF THE INVENTION

In accordance with an embodiment of the invention a lean-burn engineexhaust stream, often at a temperature in the range of about 200° C. toabout 400° C. and typically containing NO and NO₂, is treated to preparethe hot, oxygen-containing stream for selective catalytic reduction ofthe mixture of nitrogen oxides. In general, it is useful to promote theoxidation of nitrogen oxide (NO) to nitrogen dioxide (NO₂) and to addhydrocarbon and oxygenated hydrocarbon constituents to the exhaust asreductants for conversion of NO₂ to N₂. For example, a stream of ozoneproduced in ambient air plasma may be added to the exhaust gas topromote oxidation of NO, and relatively low molecular weight diesel fuelhydrocarbons (e.g., propane to dodecane) and oxygenated diesel fuelhydrocarbons (e.g., ethanol) may be added downstream of the ozoneaddition for reaction with NO₂ over a selective reduction catalyst in anexhaust reactor. The SCR stream is then discharged to the atmospherewith lower NOx content.

New bimetallic catalyst formulations are provided for improved NOxreduction of lean burn engine exhaust. In preferred embodiments of theinvention these bimetallic catalysts are used to treat exhaust in whichdiesel fuel hydrocarbons have been added as reductants (HC/SCR).Different embodiments of catalyst formulations for the HC/SCR areprovided. In one embodiment, a dual-bed catalytic reactor is usedcomprising Ba-substituted, Y-type zeolite (sometimes designated BaYherein for brevity) in the front bed and Cu and Co substituted Y-typezeolite in the rear bed (CuCoY). In a second embodiment, a dual bedselective catalytic reactor comprises Ag/alumina catalyst in the frontbed and CuCoY in the rear bed.

Y-type zeolites are a family of alumina-silicates exhibiting the crystalstructure of the mineral, faujasite. They have a three-dimensional porestructure. Sodium ion-substituted Y-type zeolites (sometimes designatedsimply as NaY) are commercially available. Some or all of the sodiumions may be substituted (by ion exchange) with other ions. In someembodiments of this invention, barium ion exchanged-Y zeolites, BaY, maybe used in one bed of the dual bed SCR reactor and copper ion exchangedand cobalt ion exchanged-Y zeolites, CuCoY, may be used in a bed of thedual bed SCR reactor.

Dual bed reactors comprising an upstream catalyst bed of particles ofalumina-supported silver, or particles of BaY, and a downstream bed ofparticles of a suitable CuCoY composition are effective in the selectivecatalytic reduction of a major portion of the NOx content of the exhauststream from a diesel engine or other hydrocarbon-fueled engine producingNOx in an oxygen-rich exhaust stream.

Other embodiments and advantages of the invention will be apparent fromthe following illustrative examples. In describing these illustrativeembodiments of the invention, reference will be made to drawings orgraphs described in the next section of this specification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flow diagram of a simulated diesel exhaustcomposition stream flowing through a dual bed reactor for reduction ofNOx to nitrogen. In the gas flow system, dodecane is added as anillustrative hydrocarbon fuel reductant and ethanol as a representativeoxygenated hydrocarbon fuel reductant. Combinations of selectivereduction catalysts, including bimetal catalysts in accordance withembodiments of this disclosure, are used in a flow-through tubularelectric furnace containing catalyst beds as a dual bed reactor.

FIG. 2 is a graph of percent conversion of NOx in a lean exhaustenvironment versus reaction temperature (° C.) for different catalystformulations for a dual bed reactor using dodecane as a reductant in thesimulated diesel exhaust. In these tests the dual bed reactor containedBaY or KY in the front bed and CuY, CuCoY, or CoCuY in the rear bed.

FIG. 3A is a graph comparing the percent conversion of dodecane versusreaction temperature (° C.) in simulated exhaust streams containingdodecane and NO₂ using BaY+Cu(2)Co(1)Y (filled circle data points) orBaY+CuY (unfilled circle data points) as a reduction catalyst. FIG. 3Bis a graph presenting the percent conversion of NOx versus reactiontemperature (° C.) in the same exhaust streams.

FIG. 4 is a pair of graphs comparing the formation of carbon-containingbyproducts (ethene-unfilled circles, formaldehyde-unfilled squares, andacetaldehyde-unfilled triangles) versus reaction temperature (° C.) in asimulated diesel exhaust stream comprising NO₂ and dodecane using a dualbed reactor of BaY+CuY (FIG. 4A) and BaY+Cu(2)Co(1)Y (FIG. 4B).

FIG. 5 is a pair of graphs comparing the formation ofnitrogen-containing byproducts (ammonia-unfilled circles, HCN-unfilledsquares, and N₂O-unfilled triangles) versus reaction temperature (° C.)in a stream comprising NO₂ and dodecane using a dual bed reactor ofBaY+CuY (FIG. 5A) and BaY+Cu(2)Co(1)Y (FIG. 5B).

FIG. 6 is a pair of graphs comparing the percent conversion of dodecaneor ethanol (FIG. 6A) versus reaction temperature (° C.) in streams ofNO₂+dodecane+ethanol in the dual-bed reduction reactor of BaY+CuCoY orin a reduction reactor using a single bed containing a physical mixtureof BaY and CuCoY. FIG. 6B compares the conversion of NOx versus reactiontemperature (° C.) in the same feed streams.

FIG. 7 is a pair of graphs comparing the formation (in ppm) ofethene-filled circles, formaldehyde-filled squares, andacetaldehyde-filled triangles versus reaction temperature (° C.) instreams of NO₂+dodecane+ethanol in a dual-bed reduction reactor ofBaY+CuCoY (FIG. 7A) and in a reduction reactor using a single bedcontaining a physical mixture BaY and CuCoY (FIG. 7B).

FIG. 8 is a pair of graphs comparing the formation (in ppm) ofammonia-filled circles, HCN-filled squares, and N₂O-filled trianglesversus reaction temperature (° C.) in streams of NO₂+dodecane+ethanol ina dual-bed reduction reactor of BaY+CuCoY (FIG. 8A) and in a reductionreactor using a single bed containing a physical mixture BaY and CuCoY(FIG. 8B).

FIG. 9A is a graph of percent conversion of NOx over an Ag/aluminacatalyst versus reaction temperature (° C.) of a feed stream of 200 ppmNO, with a C₁/NOx=8 (ethanol:dodecane=1.1 in C₁) and at a space velocityof 60,000/h. The conversion of NOx is presented in the filled circledata points and the conversion of NOx to N₂ is presented in the filledtriangle data points.

FIG. 9B is a graph of formation (ppm) of NH₃ (filled circles), HCN(filled triangles), and N₂O (filled squares) versus reaction temperature(° C.) of the feed stream and catalyst of FIG. 9A.

FIG. 10 is a bar graph comparing NOx conversion to N₂ (%) versuscatalyst temperature (250° C., 300° C., 350° C.) of a feed stream of 200ppm NO, with a C₁/NOx=8 (ethanol:dodecane=1.1 in C₁) and at a spacevelocity of 60,000/h over a single bed catalyst of Ag/alumina (bar data)and over a dual bed catalyst of Ag/alumina (front bed)+CuCoY (rear bed)

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The exhaust from a diesel engine often contains carbon particulates,unburned hydrocarbons (HC), and carbon monoxide in addition to NOxconstituents. The exhaust may be treated to remove or burn theparticulates and/or to oxidize HC and carbon monoxide to carbon dioxidebefore the exhaust is treated to chemically reduce NOx content tonitrogen.

The NOx-containing exhaust from an engine is further prepared forselective catalytic reduction of nitrogen oxides. Ozone may be added tothe exhaust to promote the oxidation of NO to NO₂. In some embodimentsof the invention the ozone is formed by passing a stream of ambient airthrough a suitable nonthermal plasma generator and a plasma stream ofair, ozone, and other oxygen-containing species added to the exhaust.Then a hydrocarbon reductant is added to the exhaust. Suchhydrocarbon(s) may be fractionated from diesel fuel drawn from on-boardstorage and contained in a suitable processing container or vessel.Ambient air may be bubbled up through the hydrocarbon fuel to striplower molecular weight hydrocarbons from the diesel fuel (or otherhydrocarbon-containing fuel). In practices of the invention the termdiesel fuel hydrocarbons may include hydrocarbons that have beenoxygenated, such as ethanol. Such hydrocarbons and oxygenatedhydrocarbons may be produced using a stream of ambient air first passedthrough a non-thermal plasma reactor to produce a mixture of ozone, air,and possibly other oxygen species, and the ozone-containing streampassed through the fuel to strip hydrocarbons and to produce someoxygenated hydrocarbons. The exhaust now contains NO₂ and hydrocarbons.The carbon content of the reformed fuel may be normalized in terms ofmolar methane (C₁). In general, the requirement for reformed diesel fuelconstituents increases with increased NOx content in the exhaust andincreased exhaust temperature (catalytic reactor temperature). Forexample, about eight moles of reformed hydrocarbon (normalized as C₁)may be required per mole of normalized NOx at a catalyst temperature of200° C. for efficient NOx removal. The required amount of ozonedecreases with an increase of the catalyst temperature. Thus, the ozonerequirement is greatest at catalytic reactor temperatures of about150-200° C. and decreases to zero at reactor temperatures of 350-400° C.

In accordance with an embodiment of the invention, new bimetalliccatalyst formulations are provided for improved NOx reduction in a leanburn engine exhaust stream.

The new catalyst material comprises formulations of copper ion-exchangedand cobalt ion-exchanged Y zeolites. Application of this catalyst forthe HC/SCR is illustrated in this specification in two different ways:First, using a catalytic reactor made of a dual-bed catalyst containingBaY in the front bed and CuCoY in the rear bed, and second, using adual-bed catalyst containing Ag/alumina in the front bed and CuCoY inthe rear bed.

In the following illustrations, which include many comparativereactions, BaY and KY catalysts were prepared from NaY with a Si/Alratio of 2.6 obtained from Zeolyst Inc. (CBV100) by a standardion-exchange process using aqueous solutions of Ba(NO₃)₂ and KNO₃ asprecursors, respectively. The ion exchange process was repeated threetimes for BaY and twice for KY, followed by drying at 100° C. overnightand calcining in air at 500° C. for 5 h.

Both CuY and CuCoY catalysts were also prepared from NaY obtained fromZeolyst Inc. (CBV100) by a standard sequential ion-exchange process—Cuion exchange first, followed by Co ion exchange—using aqueous solutionsof Cu(NO₃)₂ and Co(NO₃)₂ as precursors. In each ion-exchange step toprepare the CuCoY catalyst, 0.01M of each metal precursor was firstdissolved in 1 L of deionized water, and then 10 gram of NaY zeolite wasadded for the metal-ion exchange. The ion exchange process for Cu wasrepeated up to 3 times in order to obtain the optimum exchange level,while the Co exchange process was done only once for all samples. Theion-exchanged CuCoY catalysts were dried at 100° C. overnight, followedby calcination at 500° C. for 5 h.

The Ag/alumina catalyst containing 2 wt. % Ag was prepared on γ-aluminapowder obtained from Sasol (Catalox SBA-200) by the incipient wetnessmethod using AgNO₃ as the precursor. The impregnated Ag/alumina catalystpowder was dried at 110° C. overnight followed by calcination in air at500° C. for 10 h. The calcined catalyst powder samples were compressedunder 10 ton pressure to form a wafer. After annealing at 500° C. for 20h under atmospheric conditions, the wafer was crushed and screened to20-40 mesh sizes before being packed into the reactor.

Prior to activity measurements, the packed-bed reactor was pretreated at500° C. for 1 h under the standard reaction conditions as listed inTable 2.

Improvement of NOx reduction performance by an average NOx conversion of15% was achieved using a new catalyst formulation (BaY+CuCoY) in labreactor experiments over a wide temperature range of 250-450° C. usingsimulated diesel engine exhaust compositions, compared with thebenchmark (BaY+CuY) catalyst (FIG. 2). When the new bimetallic catalyst(CuCoY) was used in combination with the conventional Ag/aluminacatalyst in a dual-bed mode (Ag/alumina+CuCoY), improvements in NOxconversion of 20-30% were achieved over the temperature range of250-350° C. (FIG. 10).

Listed in Table 1 are metal contents contained in the new catalystformulations prepared and tested in this work, where the notation ofM1(i) M2(j)Y indicates that M1, M2 are is the first and second metalions, respectively, exchanged sequentially onto the NaY-zeolitesubstrate. The numbers i and j in the parenthesis indicate the number ofrepetitions during the ion exchange processes for M1 and M2,respectively. For example, Cu(3)Co(1)Y denotes that the Cu-ion was firstexchanged onto the NaY zeolite via three repeated ion-exchange processesfollowed by the Co-ion exchange.

TABLE 1 Metal Content (wt. %) Catalysts Ba K Cu Co Na Ag Ba(3)Y 6-10K(2)Y 8 <0.5 Cu(2)Y 7.3 Ag/alumina 2.0 Co(1)Cu(1)Y 3.0 2.1 2.2Cu(1)Co(1)Y 3.2 2.6 1.7 Cu(2)Co(1)Y 5 1.7 1.5 Cu(3)Co(1)Y 6 1.7 1.2

Listed in Table 2 are the experimental conditions used to demonstratethe NOx conversion performance of the new catalysts in the lab reactorexperiments.

TABLE 2 Standard Experimental Conditions Benchmark catalysts: single-bedcatalyst (Ag/alumina); dual-bed catalyst (BaY + CuY). Catalyst bedconfigurations: dual-bed catalysts (BaY + CuCoY) & (Ag/alumina + CuCoY);physical mixture catalyst (BaY − CuCoY). Catalyst temperature: 150-500°C. Catalyst powder: 20/40 mesh Total flow rate: 200 cc/min Reactor spacevelocity: 30,000-60,000 h-1 Feed gas composition: 200 ppm NO or NO₂, 6%O₂, 2.5% H₂O, balance N₂ C₁/N = 8, composed of 133 ppm Dodecane, or 67ppm Dodecane + 400 ppm Ethanol [D/E = 1 on C₁ basis]

FIG. 1 shows the schematic flow diagram of the laboratory reactor systemused to evaluate the NOx reduction performance of various catalysts inthe Diesel/SCR process where simulated diesel fuel was used as thereductant. Catalytic activity was measured between 150 and 500° C. in apacked-bed flow reactor at atmospheric pressure. The feed reactant flowto the catalyst contained 200 ppm NO₂, 6% O₂, 2.5% H₂O, 133 ppm dodecane(or a mixture of 67 ppm dodecane and 400 ppm ethanol) and balance N₂.Note that either dodecane alone or the mixture of dodecane and ethanolmaintains the carbon/NOx feed ratio of 8. Here dodecane was used as arepresentative diesel fuel hydrocarbon, while the mixture of dodecaneand ethanol was used as a representative mixture of diesel fuelhydrocarbon and an oxygenated hydrocarbon. Both the feed and the productcomposition to and from the catalytic reactor were analyzed by an FTIR.

FIG. 2 compares the NOx conversion performances of different catalystformulations for a dual-bed reactor containing BaY or KY in the frontbed and CuY, CuCoY or CoCuY in the rear bed, when dodecane was used asthe reductant for NO₂ reduction. The (BaY+CuY) dual-bed catalyst wasused as the benchmark catalyst, since its good activity has beendemonstrated previously in a plasma-catalyst system for NOx reduction(see U.S. Pat. No. 7,090,811). The NOx conversion activity of the(KY+CuCoY) catalyst is better than that of the (KY+CoCuY) catalyst,indicating that the order of ion exchange with the first Cu-ion exchangefollowed by the Co ion exchange is preferable to the reverse order. Notethat both of these two catalysts with KY in the front bed are not asgood as the bench mark (BaY+CuY) catalyst in terms of their NOxconversion performance. On the other hand, the [BaY+Cu(1)Co(1)Y] and[BaY+Cu(2)Co(1)Y] catalysts show better NOx conversion performance thanthe benchmark catalyst over the catalyst temperature range of 150-450°C. Particularly noteworthy is the fact that the NOx conversion activityof the latter catalyst which was ion-exchanged twice with Cu is betterthan that of the former which was ion-exchanged only once with Cu,indicating that the multiple ion-exchange is preferable for the Cu-ionexchange. In a series of more experiments, it was revealed that morethan two repetitions of the Cu-ion exchange process and more than asingle ion exchange of Co did not make any further improvement in theNOx conversion performance of the catalysts. The data in FIG. 2 clearlydemonstrate that BaY is preferable to KY in the front bed, while CuCoYis preferable to CuY in the rear bed in a dual-bed catalyst system.Among those new catalyst formulations developed in this work, the[BaY+Cu(2)Co(1)Y] dual-bed catalyst proved to be the best and itsperformance for NOx reduction was essentially the same as that of[BaY+Cu(3)Co(1)Y].

FIG. 3 compares a new catalyst formulation and the benchmark catalystformulation for the conversion of both HC and NOx when dodecane was usedas the reductant for NO₂ reduction. Above 250° C., it is interesting tosee that the NOx conversion performance of the new catalyst formulationcontaining the bimetallic CuCoY catalyst in the rear bed is much better(about 15% improvement) than that of the benchmark catalyst containingCuY catalyst in the rear bed, even though the HC conversion performanceis essentially the same for both catalysts. Below 200° C., the newcatalyst formulation is better than the benchmark catalyst for the NOxconversion, whereas this trend reverses for the HC conversion. Bothfindings put together, it is obvious that the hydrocarbon reductant(dodecane in this case) is more efficiently utilized for NOx reductionover the new catalyst (BaY+CuCoY) than over the benchmark catalyst(BaY+CuY).

FIG. 4 compares the formation of carbon-containing byproducts in thedual-bed catalyst containing the [BaY+Cu(2)Co(1)Y] catalysts with thatin the benchmark (BaY+CuY) catalyst, when dodecane was used as thereductant for NO₂ reduction. It reveals that the new catalyst containingthe bimetallic CuCoY catalyst produces more reaction intermediates suchas ethene (C₂H₄, ethylene) and aldehydes than the benchmark catalyst,which are known to be effective reductants for NOx reduction over basemetal catalysts such as Cu and Co. This may explain why the new catalystformulation is more effective for NOx conversion than the benchmarkcatalyst formulation, as shown in FIG. 3.

FIG. 5 compares N-containing byproducts produced during the NOxreduction process over the new catalyst (BaY+CuCoY) and the benchmarkcatalyst (BaY+CuY), when dodecane was used as the reductant for NO₂reduction. It is clear that the new catalyst formulation promotes theformation of HCN compared to the benchmark catalyst, while N₂O formationis slightly shifted to higher temperatures. For both catalysts, NH₃formation is negligible. In light of the data in FIGS. 3 and 5 it can beconcluded that the new dual-bed (BaY+CuCoY) catalyst improves the NOxconversion performance by about 15% over the catalyst temperature rangeof 250-450° C. compared to the benchmark (BaY+CuY) catalyst whileproducing more HCN, when dodecane was used as the NOx reductant.

It has been shown that diesel fuel can be reformed by treating with airplasma to produce oxygenated hydrocarbons (OHC's) such as ethanol. Inorder to simulate the reformed diesel fuel, ethanol was added tododecane to make a gaseous mixture of 67 ppm dodecane and 400 ppmethanol while maintaining the C₁/NOx feed ratio at 8. FIG. 6 shows theeffect of catalyst bed configuration when a mixture of dodecane andethanol was used as the reductant for the new catalyst formulation. Oneconfiguration is the dual-bed [BaY+Cu(3)Co(1)Y] designated as BaY+CuCoY,and the other is a uniform physical mixture of BaY and Cu(3)Co(1)Ydesignated as BaY—CuCoY. As shown in FIG. 6A, ethanol conversion is muchhigher than dodecane conversion, and both conversions are essentiallythe same for both catalyst configurations. FIG. 6B shows that the NOxconversion over the dual-bed configuration is much better than that overthe physical mixture, even though the hydrocarbon reductant conversionsare the same for both catalysts as shown in FIG. 6A.

FIG. 7 compares the formation of carbon-containing byproducts in thedual-bed catalyst configuration containing the [BaY+Cu(3)Co(1)Y]catalysts with that in the physical mixture, when a mixture of dodecaneand ethanol was used as the NOx reductant. It shows that the dual-bedconfiguration produces much more reaction intermediates such as etheneand aldehydes than the physical mixture. Note that those hydrocarbonintermediates are known to be effective reductants for NOx reductionover base metal catalysts such as Cu and Co. This may explain why thedual-bed configuration is much more effective for NOx conversion thanthe physical mixture, as shown in FIG. 6.

FIG. 8 compares N-containing byproducts produced during the NOxreduction process over the dual-bed configuration and the physicalmixture, when a mixture of dodecane and ethanol was used as the NOxreductant. It shows that the amount of N-containing byproduct formationdepends strongly on catalyst temperature for both configurations.However, the dual-bed configuration reduces the total amount ofundesirable N-containing byproducts formation such as ammonia and N₂Ocompared with the physical mixture over the catalyst temperature rangeof 150-450° C., while HCN formation is negligible for bothconfigurations. Comparison of FIG. 5 b with FIG. 8 a indicates that theuse of a mixture of ethanol and dodecane as the reductant greatlyreduces the formation of undesirable N-containing byproducts such as N₂Oand HCN over the (BaY+CuCoY) catalyst, compared with the use of dodecaneas the reductant. In light of the data in FIGS. 6 and 8 it is concludedthat the dual-bed configuration is preferable to the physical mixture ofthe BaY and CuCoY catalysts for NOx reduction using a mixture ofhydrocarbon and alcohol as the NOx reductant.

The enhanced NOx reduction activity of the dual-bed (BaY+CuCoY) catalystcompared with that of the benchmark (BaY+CuY) catalyst may be explainedby the synergistic catalytic effect caused by electron transfer betweenCu and Co. It is well established that the active catalytic sites of Cuand Co for NOx reduction are Cu²⁺ and Co²⁺ sites. The redox potential ofCu¹⁺/Cu²⁺ is 0.15V, while that of Co²⁺/Co³⁺ is 1.82V. This means Co hasa stronger affinity for an electron than Cu, which results in electrontransfer from Cu to Co. That is, Cu¹⁺ donates an electron to becomeCu²⁺, while Co³⁺ accepts an electron to become Co²⁺. Thus, Cu¹⁺ acts asan electron donor, while Co³⁺ acts as an electron acceptor. This way,Cu²⁺ helps to stabilize Co²⁺, leading to the synergistic catalyticeffect for NOx reduction on the catalytically active Cu²⁺ and Co²⁺ sites

The dual-bed catalysts containing the BaY catalyst in the front bed andthe bimetallic CuCoY catalyst in the rear bed disclosed in thisinvention is suitable for an engine exhaust stream that contains NO₂ asthe major NOx species. (Note that NOx refers to both NO and NO₂.) Sincethe major NOx species in a typical lean-burn engine exhaust gas such asdiesel engine exhaust is not NO₂ but NO, an oxidation catalyst or aplasma reactor may be used in practice to convert NO to NO₂ in theexhaust. It is also important to note that a mixture of hydrocarbons andoxygenated hydrocarbons such as alcohols can be produced from raw dieselfuel using a fuel reformer assisted by air plasma [Cho, U.S. Pat. No.6,957,528-B1, U.S. Pat. No. 7,093,429; Cho and Olson, U.S. PatentApplication Publication 2006/0283175].

Presented in FIG. 9 is the NOx conversion performance of the Ag/aluminacatalyst along with N-containing byproducts such as NH₃, HCN and N₂O. Itis noteworthy in FIG. 9A that the total NOx conversion of 100% isachieved in the catalyst temperature range between 225° C. and 350° C.,even though the true NOx conversion to N₂ is much lower at around 60%.This difference can be explained by the large amount of NH₃ formation inthis temperature range as shown in FIG. 9B. That is, NOx completelydisappears during the HC/SCR process over Ag/alumina at 225-350° C., butproduces NH₃ at the same time.

FIG. 10 compares the NOx reduction activity of Ag/alumina in asingle-bed configuration with that of (Ag/alumina+CuCoY) in a dual-bedconfiguration with the Ag/alumina in the first bed and the CuCoY in thesecond bed, while keeping the overall gas space velocity the same forboth configurations. FIG. 10 clearly demonstrates that the CuCoYcatalyst in the rear bed enhances the overall NOx reduction activity ofthe system by 20-30% over 250-350° C., which can be explained by thesuperior activity of CuCoY for NH₃ oxidation to make N₂ compared toAg/alumina.

It is important to note that the bimetallic catalyst (CuCoY) disclosedin this invention can be used alone to replace CuY (as shown in FIG. 2)or in combination with other HC/SCR catalysts such as Ag/alumina (asshown in FIG. 10) in order to improve the NOx reduction activity of theHC/SCR catalysts.

The practice of this selective reduction process for NOx content of alean engine exhaust may be conducted by adding a suitable quantity of anoxidant, such as ozone, to the hot exhaust for oxidation of a desiredportion of its NO content to NO₂. The practice of this selectivereduction process may also be conducted by adding suitable relativelylow molecular weight, gaseous or volatile hydrocarbons (e.g., C₂ toabout C₁₄) and/or suitable oxygenated hydrocarbons (e.g., formaldehyde,ethanol, and acetaldehyde) to the exhaust stream as reductants for NO₂.And the reductants may be added in amounts providing, for example, asuitable ratio of carbon to nitrogen for reduction of thenitrogen-containing species of the exhaust. In illustrative embodimentsof the specification, the carbon-to-nitrogen mole ratio was about eight.

In some embodiments of the invention, it may be desired to use anon-thermal hyperplasma reactor for generating ozone in a stream of airand for reforming fractionated diesel fuel in a stream of air. Anexample of such a non-thermal plasma reactor is illustrated in FIG. 3 ofeach of U.S. Pat. Nos. 6,957,528, 7,093,429, and U.S. Patent ApplicationPublication 2006/0283175, and the related text in the specifications ofthese documents. Such a reactor may be sized and electrically poweredfor producing a suitable quantity of ozone and/or of fractionated orreformed diesel fuel for an exhaust stream. A plasma reactor may belocated, for example, in or near the engine compartment of a vehicle anda stream of ambient air may be supplied by a suitable blower to theplasma reactor. The ozone-containing outlet stream from the plasmareactor may be apportioned for direct addition of ozone to the exhaustand for bubbling through a vessel containing a volume of diesel fuel forfractionating lower molecular weight hydrocarbons from the fuel volumeand for reforming some of the hydrocarbon(s) to oxygenated hydrocarbons.These drawing figures and related text from these patents areincorporated herein by reference for their disclosures and use of anon-thermal hyperplasma reactor.

In general, such a nonthermal plasma reactor comprises a cylindricaltubular dielectric body suitably made of a ceramic material. The reactorhas two electrodes, a high voltage electrode and a ground electrode,separated by the tubular dielectric body and an air gap. The highvoltage electrode is a straight rod placed along the centrallongitudinal axis of the tube. The ground electrode is a conducting wirewound around the tubular dielectric body in a helical pattern. Thehelical ground electrode in combination with the axial high voltageelectrode provides intertwined helical regions of active and passiveelectric fields along the length (longitudinal axis) of the reactor. Thehelical active electric field around the ground electrode is highlyfocused for effective plasma generation for ozone formation frommolecular oxygen. The reactor is effectively a hyperplasma generator forpurposes of the practice of this invention.

A high voltage, high frequency electrical potential is applied to theend leads to the center electrode. The helical outer ground electrode isgrounded. In the operation of the nonthermal hyperplasma reactor an airstream (which may include re-circulated exhaust gas) flows through theinlet of reactor around the center electrode and within the dielectrictube and out of the exit end. The electrical potential applied to thecenter electrode generates the above described active and passiveelectric fields within the reactor. These intertwined high potential,high frequency fields are very effective in generating reactive ozoneand oxygen atoms, radicals, and ion containing species within theflowing air stream in the air gap. This ozone-containing air streamleaves the nonthermal plasma reactor for addition to the exhaust streamand, optionally, for treatment of a volume of diesel fuel to fractionateand reform it.

Practices of the invention have been illustrated by some specificexamples and embodiments which are presented to illustrate the inventionand not to limit its scope. Clearly, many other embodiments of theinvention may be devised by one skilled in the art.

1-10. (canceled)
 11. A method of making a catalyst for reducing nitrogenoxides in an exhaust stream from an engine operating in a lean burnmode, the exhaust stream comprising NO, NO₂, oxygen, water, andnitrogen; the method comprising: exchanging copper ions in water forother base metal ions in a Y type zeolite and exchanging cobalt ions inwater for other base metal ions in a Y type zeolite.
 12. A method ofmaking a catalyst for reducing nitrogen oxides as recited in claim 11 inwhich copper ions in water are first exchanged with a first portion ofother base metal ions and cobalt ions are subsequently exchanged with asecond portion of base metal ions.
 13. A method of making a catalyst ofreducing nitrogen oxides as recited in claim 12 in which copper ions inwater are twice exchanged for base metal ions before cobalt ions areexchanged for the other base metal ions in the Y type zeolite.
 14. Amethod of making a catalyst of reducing nitrogen oxides as recited inclaim 12 in which cobalt ions are exchanged a single time for other basemetal ions in the Y type zeolite.
 15. A method of making a catalyst forreducing nitrogen oxides as recited in claim 11 in which the other basemetal ions are sodium ions.
 16. A method of making a catalyst as recitedin claim 11 in which the ion exchange steps are conducted to obtaincatalyst particles comprising, by weight, about three to about sixpercent copper and about one and one-half percent to about three percentcobalt.